Colour is the heart-beat of commercial printing and publishing. Accurate colour both on the screen, on the colour printer, proofer and Colour is the heart-beat of commercial printing and publishing. Accurate colour both on the screen, on the colour printer, proofer and on press, is what we strive for, but in order to manage colour reproduction we need appropriate measuring devices. So this article is about the five big Ws of colour management: When to measure colours? Where? Why? and With What?

With what?
Starting backwards, we need to know what measuring devices are suitable for which types of measurements. Densitometres, both for transmission (reading film) and reflection (reading print on paper and other substrates) are well known in the graphic arts industry, and hopefully in use daily in most printing plants worldwide. But the densitometer is actually totally colour blind. Inside a densitometer is a light source and a selection of filters, which help the control software identify the printing colours to measure. To be able to read out a density value for cyan, the complementary colour red is used in the filter, for example. But we don't know if this is the accurate colour “cyan,” we only know that it's something “cyanish,” with a density (greyscale level) of let's say 1.4 D that we look at. So while densitometers are very useful and important, they have their limitations. What they can do, however, is to be programmed to also calculate the screen density, useful when calculating the dot gain in print (in the ISO standards dot gain is referred to as Tone Value Increase, TVI).

To actually measure true colours, you need at least a colorimeter. Like a densitometer it also contains a lightsource (unless it's only supposed to measure emitted light), and filters. A colorimeter is amongst the cheapest of devices for colour measurements, and is commonly used to calibrate and characterise monitors. A colorimeter in some sense acts as a densitometer, but it reads and expresses values in CIELab. The CIELab colour space is the preferred standard when expressing absolute colour values, and is used in ICC-profiles and the latest revision of print standards like the ISO 12647.

A more accurate colour measurement device (and therefore more expensive) is the spectrophotometer. Like the name suggests it measures the spectral composition of the emitted or reflected light. A spectrophotometer is by far the most flexible device, since it also can be used as an advanced densitometer. Knowing the intensities over the whole range of visible wavelengths (often also including the nearly visible wavelengths like ultra violet), a better and more accurate analysis and definition of the colours can be made.

Finally we may need a dotmeter to be able to measure screen density on the printing plate. Optimists have tried using conventional densitometers for this, but the often low contrast especially on CtP plates has been found to be problematic for “normal” densitometers. The measurements have been found to vary too much for the result to be trusted for quality assured work. The dotmeters, sometimes referred to as “digital microscopes,” are absolutely necessary when calibrating and linearising a CtP device.

Why and where?
In a colour managed workflow it's key to establish a known status for all the devices. In reality this means calibrating the device in question and, when necessary, linearising its behaviour. For a monitor this means defining at what whitepoint we want it to work, at what light intensity (brightness) and what greyscale curve it should apply (gamma curve). For this we need a colorimeter, or better, a spectrophotometer. On top of this, to evaluate the monitor's suitability for accurate soft proofing we need some evaluation software. One of the few on the market is the Ugra/Fogra software called UDACT (Ugra Display Analysis and Certify Tool).

To linearise colour printers we need at least a densitometer, but it's better to use a good spectrophotometer, since this is also necessary for the characterisation (creating ICC-profiles) process.

For the calibration and linearisation of a CtP device it's necessary to use a dotmeter. If this isn't done, we have no clue if the plates are correctly exposed and/or processed. If we don't know the dot values (tone values) on the plate, we then don't know if the TVI is reasonable and within tolerances.

In order to be able to use one standardised ICC-profile for a certain paper type, we might need to apply different TVI curves in the RIP, for the different printing presses at our printing plant. This is at least one way to achieve predictable and accurate print quality across a press fleet. But in order to do this we need to measure both the plates and the actual print, on press. Most modern press control systems use a scanning spectrophotometer at the press, or a scanning densitometer. If you use a densitometer you need to translate the target colour values according a given standard, for example the ISO 12647-2 (sheetfed offset) by using a spectrophotometer at some point. From there on, the measured colours, expressed as CIELab-values, can now be translated to “standard” density target values, in that press, for that paper.

When?
When, and how often measurements need to be taken depends on the device we are talking about, and the tolerances that you set up in your quality assurance work. Generally measurements need to be done when a new batch of ink, film or plates is delivered, or when any process parameter is radically changed. Since more and more printers have started using spectrophotometers at the printing press, they find that the ink batches can differ quite considerably. Cyan is not always the cyan we expected it to be (or what it should be according to the ISO 2846-1 standard). Same thing with plates: one batch may need quite different settings in the CtP than the previous, and this isn't discovered if proper process control isn't applied. Actually it is discovered, and with generally undesirable consequences, because it's too late to fix.

One way to automate this is to implement what is called “closed loop systems.” In newspaper production systems for automatic control of CtP-plates has been introduced by for example Agfa and Nela. The Agfa solution is called Afirma and ensures that the plates are checked for correct exposure and processing directly and automatically at or inside the CtP device.

For proofing systems and monitors it's difficult to give a general rule of thumb how often they need to be calibrated — it depends on how stable the system is in itself, and at what quality level you want to operate. An important factor is the viewing booth. Check that the light is at both the white point you have decided, normally 5000 K according to the D50 standard, and at the correct intensity, at least 1500 lux (all according to the ISO 3664 standard).

Even the measuring device needs to be calibrated now and then. The filters in both densitometers, colorimeters and spectrophotometers (possibly also dotmeters) age over time, and need to be replaced or at least recalibrated. Again, it depends on at the quality level you want to achieve, but most devices need a big overhaul at least every third or second year. Some vendors suggest every year, and X-Rite (that now includes Gretag Macbeth) even offer a system to calibrate your device against a “master device” remotely. The solution is called Net Profiler, and makes it possible for an organisation to monitor and ensure that all measuring devises are synchronised to measure in the same way, within the defined tolerances.

Challenges
Once regular measurements are made in the workflow, you soon discover that when a factor or technology is changed, the measuring device you use may not any longer be relevant and appropriate for the task. This is the situation with some of the new processless CtP plates. Some of those plates have such a low contrast in the image on plate that even some of the dotmeters have problems seeing and so measuring the dot in a stable and predictable way. Some plates in particular have been mentioned in discussion forums and articles, like for example the Fujifilm Brillia Pro-T and the Kodak Thermal Direct. But such problems are for the engineers to solve, and an upgrade of the Techkon Spectroplate to a version called All Vision is supposed to be able to read even low contrast CtP plates like the ones mentioned above.

Another challenge for the dotmeters is to correctly identify and read tone values for different screen types. Besides conventional AM and FM screens a CtP may very well produce a hybrid screen or a second generation FM screen. This needs to be taken care of in the control software for the measuring device, but even this seems to be a non-issue according to X-Rite, if you use their latest Plate Scope device. Using a special light source called Spectral Illumination Array it's supposed to read low contrast plates as well as many different screen types.

There are other challenges of course for the quality control manager, not least to convince all involved in the print and publishing workflow to establish and maintain a reasonable level of process control. But well managed and colour controlled workflow should pay off in the long run. And then we can discuss what the true colour of money really is.